1. Field of the Invention
The present invention relates to signal processing techniques. More particularly, the present invention relates to methods for processing audio signals.
2. Description of the Related Art
Two-channel phase-amplitude stereo encoding, also known as “matrixed surround encoding” or “matrix encoding”, is widely used for connecting the audio output of a video gaming system to a home theater system for multichannel surround sound reproduction, and for low-bandwidth or two-channel transmission or recording of surround sound movie soundtracks. Typically, in the gaming application, a multi-channel audio mix is computed in real time (during game play) by an interactive audio spatialization engine and down-mixed to two channels by use of a matrixed surround encoding process identical to those used for matrix encoding multi-channel movie soundtracks. As a result of the encoding-decoding process, schematically illustrated in FIG. 1A, the surround sound mix can be transmitted via a single standard stereo audio connection or via a S/PDIF coaxial or optical cable connection commonly available in current home theater equipment. The multichannel mix composed in the interactive audio rendering engine is typically obtained as a combination (mixing) of localized sound components reproducing point sources (primary sound components) and of reverberation or spatially diffuse sound components (ambient sound components).
An advantage of phase-amplitude stereo encoding compared to alternative discrete multi-channel audio data formats (such as Dolby Digital or DTS) is that the encoded data stream is a two-channel audio signal that can be played back directly (without any decoding) over standard two-channel stereo loudspeakers or headphones. For multichannel loudspeaker presentation, a matrixed surround decoder can be used to recover a multichannel signal from the matrix-encoded two-channel signal. However, with currently available time-domain matrixed surround decoders, the fidelity of the spatial reproduction typically suffers from inaccurate source loudness reproduction, inaccurate spatial reproduction, localization steering artifacts, and lack of “discreteness” (or “source separation”), when compared to direct multi-channel reproduction without matrixed surround encoding/decoding.
MPEG Surround technology enables the transmission, over one low-bit-rate digital audio connection, of a two-channel matrix-encoded signal compatible with existing commercial matrixed surround decoders, along with an auxiliary spatial information data stream that an MPEG Surround decoder utilizes in order to recover a faithful reproduction of the original discrete multi-channel mix. However, the transmission of auxiliary data along with the audio signal requires a new digital connection format incompatible with standard stereo equipment.
Another limitation of the above audio encoding-decoding technologies is their restriction to horizontal-only spatialization, their bias towards a particular multi-channel loudspeaker layout, and their reliance on the spatial audio rendering technique known as multi-channel amplitude panning. This makes these technologies non-ideal for reproduction using headphones or alternative loudspeaker layouts and spatialization techniques (such as ambisonic or binaural technologies, for instance), which are more effective than the amplitude panning technique for improved spatial audio reproduction in some listening conditions. For headphone playback, in particular, a superior listening experience could be obtained by use of binaural 3-D audio spatialization methods, also requiring only two audio transmission channels. However, due to the inclusion of head-related inter-channel delay and frequency-dependent amplitude difference cues in the encoded signal, a binaural transmission format would be unsuited to multi-channel surround sound reproduction over an extended home theater listening area.
It is desired to overcome the above limitations of existing matrixed surround encoding and decoding technology by providing more flexible and spatially accurate encoding and decoding schemes.